1. Technical Field of the Invention
The present invention relates to a controller which is used for process control, in particular, to a controller which excels in both set point value following characteristics and external disturbance suppression characteristics.
2. Prior Art
Due to an increase in process operating needs such as (1) resources saving and energy saving, (2) reduction in personnel and saving power, (3) uniformization and high quality of products, (4) safety, and (5) flexibility, process control is being required to possess an increasingly high controllability, and various means have been devised attempting to attain this objective.
In operating a plant, in particular, a continuous process or the like, if the control of the system is disturbed even by a small amount because of various factors such as changes in yield in upstream processing, various external disturbances, changes in the set point value due to optimization cascading control, and so forth, downstream processing is influenced in a chain-reaction manner. Accordingly, the fundamental principle for control is to improve the controllability of the individual control system to its extremes. Recently, there have been frequent cases in which large disturbances are generated such as changes in load during operation, along with the flexibility of plant operation and energy conservation, or there are frequent and large changes in the set point value due to optimization, cascading control, set point value control, and so forth.
In such circumstances two problems occur when implementing the above requirements, (1) attaining a desired system response against changes in a set point value, and (2) achieving sufficient feedback to handle disturbances. Prior art controllers, however, have not been configured to cope simultaneously and sufficiently with the two problems. In particular, it is desired to accomplish a satisfactory adjustment of integral time which is likely to cause inconsistency between the characteristics for following the change in the set point value and the characteristics for suppressing influences due to disturbances.